api: IOProxy const method adjustments (#4415)

With 3.0 approaching, it seems like the only time to make these changes
that I made note of a while back:

* It seems to me that `tell()` should be const because it isn't going to
modify or change the state of the IOProxy.

* It seems to me that `flush()` should NOT be const, because it probably
does change internal state.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

src/include/OpenImageIO/filesystem.h

@@ -412,7 +412,7 @@ class OIIO_UTIL_API IOProxy {
     virtual const char* proxytype () const = 0;
     virtual void close () { }
     virtual bool opened () const { return mode() != Closed; }
-    virtual int64_t tell () { return m_pos; }
+    virtual int64_t tell() const { return m_pos; }
     // Seek to the position, returning true on success, false on failure.
     // Note the difference between this and std::fseek() which returns 0 on
     // success, and -1 on failure.
@@ -440,7 +440,7 @@ class OIIO_UTIL_API IOProxy {
 
     // Return the total size of the proxy data, in bytes.
     virtual size_t size () const { return 0; }
-    virtual void flush () const { }
+    virtual void flush() { }
 
     Mode mode () const { return m_mode; }
     const std::string& filename () const { return m_filename; }
@@ -491,7 +491,7 @@ class OIIO_UTIL_API IOFile : public IOProxy {
     size_t pread(void* buf, size_t size, int64_t offset) override;
     size_t pwrite(const void* buf, size_t size, int64_t offset) override;
     size_t size() const override;
-    void flush() const override;
+    void flush() override;
 
     // Access the FILE*
     FILE* handle() const { return m_file; }

src/libutil/filesystem.cpp

@@ -1309,7 +1309,7 @@ Filesystem::IOFile::size() const
 }
 
 void
-Filesystem::IOFile::flush() const
+Filesystem::IOFile::flush()
 {
     if (m_file)
         fflush(m_file);